Publication gravure printing inks generally comprise mixtures of a resinous binder, a colorant, and a solvent. In ink formulations, resinous binders are commonly used to provide certain physical properties to the ink which permit processing and use of the ink in publication gravure printing presses.
in a typical ink formulation, the colorant is provided in the form of a base grind which is a concentrated dispersion of colorant, typically in the form of a pigment, and a metal resinate vehicle. The base grind is extended, i.e., "let down" through the use solutions of resinous binders which adjust the base grind to a desired concentration in the ink.
These solutions, also known as "let-down vehicles", typically comprise metal resinates in a solvent. Amongst the desired properties that let-down vehicles impart to ink formulation are improved rub resistance, drying and blocking resistance.
A key property of a let-down vehicle, and one which in the past has proven difficult or expensive to provide, is the let-down vehicle's "solvent dilution ratio", often called merely its "dilution" or "dilutability". Dilution is defined herein as a measure of the vehicle's relative insensitivity to viscosity reduction as more solvent is added to an ink. Inks based on vehicles with high dilution may be cut with more solvent to bring the ink to a desired print viscosity. This property permits production of more economical inks since solvent is a relatively inexpensive component in inks.
It is desirable that the let-down vehicle contained in the ink formulation be effective in both solvents with a high aromatic content, typically toluene, as well as those solvents with a relatively high aliphatic (i.e. low aromatic) content, since substantial amounts of both types of solvents are used in the printing industry.
Hydrocarbon solvent solutions of metal resinates (e.g. calcium, zinc, magnesium, sodium, potassium, lithium, and/or aluminum salts of rosin or modified rosin) while inexpensive cannot normally be used exclusively to obtain let-down vehicles with the desired dilution. The limited molecular weights which are obtained by the reaction of metals with modified rosins results in vehicles with relatively low dilution.
High molecular weight polymeric additives have been added to metal resinate solutions to increase the dilution of the vehicle. However, these additives tend to increase the viscosity of the metal resinate solutions beyond the capabilities of ink manufacturer to pump the solutions. Such additives have limited utility in the printing industry.
Therefore, it is desirable to find a way to impart appropriately high dilution to metal resinate solutions, without causing too great an increase in viscosity, and without seriously impairing any of the other performance-related properties of the vehicle. These include, for instance, proper ink performance in terms of gloss, drying, blocking resistance, resistance to viscosity change upon storage, and resistance to bronzing and loss of pigment color strength.
There have been numerous efforts in the past to prepare metal resinate vehicles as binders with high dilutions. Some of these efforts have been successful in preparing metal resinate vehicles with high dilutions, however these metal resinate vehicles suffer from various disadvantages.
U.S. Pat. No. 5,026,753 to West et al. teaches addition of various carboxylated polystyrene resins to metal resinate vehicles, thereby imparting improved dilution to the vehicle without incurring an excessively high vehicle viscosity. This patent teaches that the most preferred polymers are styrene-maleic anhydride (SMA) copolymers. However, SMA copolymers are not effective in the full range of publication gravure solvents presently in use in the printing industry. In particular, SMA copolymers have poor solubility in vehicles employing solvents with aromatic content below about 85%. The use of these solvents is extensive in the industry, and the inability to use the SMA resins in these solvents is a distinct disadvantage. In addition, there is a degree of viscosity instability for resinate vehicles using this technology.
Similarly, U.S. Pat. No. 5,026,754 to Pavlin teaches dilution improvement by addition of various tertiary amine-containing polymers to metal resinate vehicles. However, polymers taught in this patent are not commercially available, and thus not feasible from a commercial perspective.
U.S. Pat. Nos. 5,189,089 and 5,098,479 to Hutter teach the use of amine-containing polymeric additives, but these patents also teach that these amine-containing polymeric additives are of use only in the case of zinc-containing metal resinates. Although many commercial metal resinate vehicles contain zinc, not all do. The presence of zinc is well known in the printing art to create stability problems for yellow inks, therefore the use of zinc containing resinate vehicles is to be avoided in applications containing yellow inks. Therefore, the teachings of these patents are not applicable to the full range of commercial resinate formulations.
U.S. Pat. No. 5,189,089 to Hollis et al. teaches use of ethylene-vinyl acetate copolymers as dilution enhancers for metal resinate gravure vehicles. However, the resulting dilution is relatively low when compared to other metal resinate let-down vehicles. More importantly, ethylene-vinyl acetate copolymers are not effective in vehicles in which the solvent is pure toluene, or has a relatively high toluene content. This eliminates their use in a large percentage of potential applications.
U.S. Pat. No. 4,244,866 to Schefbauer teaches use of alpha olefin/maleic anhydride copolymers and partial esters thereof, but does not teach any improvement in dilution from their use. Moreover, the low molecular weight, high acid number polymers described therein need to be used in relatively high amounts, around 10%, and, in fact, are not readily soluble in toluene which is a solvent commonly used for publication gravure printing. Thus, it is necessary to use methyl ethyl ketone, a cosolvent not employed in the industry.
Amongst the objectives of the instant invention is provision of a binder which may be used in either solvent systems having high-aromatic content and solvent systems having low-aromatic content, without requiring use of ketones or other cosolvents not generally accepted for use in the industry. Another objective of the instant invention is provision of a dilution-enhancing polymer whose efficacy is not dependent on the presence of zinc, a known cause of instability in inks having yellow pigments. Another objective of the instant invention is provision of a dilution-enhancing polymer which effectively imparts appropriately high dilution to metal resinate solutions in relatively small amounts, thereby saving cost and reducing potential for reduction of desirable properties of the metal resinate component.